An engine of a hybrid vehicle may be started by rotating the engine with a high voltage electric machine and supplying spark and fuel to the engine. The electric machine may rotate the engine to a desired engine idle speed before the engine is supplied with spark and fuel. Alternatively, the electric machine may rotate the engine at a cranking speed (e.g., 200 RPM) at which time spark and fuel may be supplied to the engine. Some hybrid vehicles include a driveline disconnect clutch that is positioned between the engine and the high voltage electric machine. The driveline disconnect clutch allows the high voltage electric machine to operate independently from the engine. Consequently, the vehicle has the capability of being propelled solely via the high voltage electric machine. However, the driveline disconnect clutch may make it more difficult to start the engine at cold ambient temperatures because the driveline disconnect clutch requires a pump to supply it pressurized working fluid in order to close so that the engine may be rotated. Consequently, the high voltage electric machine, engine, and the working fluid pump may have to be rotated by the high voltage electric machine at a time when the battery that provides power to the high voltage electric machine may exhibit reduced output power due at least in part to a lower battery discharge limit.
The inventors herein have recognized the above-mentioned disadvantages and have developed a driveline method, comprising: adjusting an electric machine to a first speed in response to a request to start an engine; adjusting the electric machine to a second speed after achieving the first speed in response to the request to start the engine; and starting an engine via closing a driveline disconnect clutch while or after transitioning the electric machine to the second speed in response to the request to start the engine.
By rotating a driveline at a first speed before lowering driveline speed and cranking an engine at a second speed, it may be possible to provide the technical result of starting the engine at lower temperatures where a battery supplying electrical power to the electric machine may have reduced discharge power limits (e.g., kW). For example, an electric machine may be rotated at a first higher speed to provide working fluid pressure capable of closing a driveline disconnect clutch. After the desired working fluid pressure is achieved, the electric machine speed may be reduced to a speed where the engine may be cranked with less torque than if the engine were cranked at higher speeds. Consequently, a desired working fluid pressure to close a driveline disconnect clutch may be achieved, and the engine may be cranked without the reduced energy storage device discharge limits being exceeded.
The present description may provide several advantages. In particular, the approach may allow an engine to be started during conditions of lower energy storage device discharge limits. Further, the approach may provide alternative ways to start an engine of a hybrid vehicle. Further still, the approach may reduce the possibility of vehicle component degradation.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.